Let's face it, iOS is fun. It feels like a new phone once you've installed it. Things zip, zoom, and ooze around, and most things feel more spacious and clean.
My favorite bits of iOS 7: I like how, after typing a text message and hitting send, the text bubble sort of spills from the entry box to the field above. It is a small, organic touch. I also love the slide finger down to do a global search. I've always found global searches to be key to being fast and effective; it's the genius of google (one search box) and a launcher like LaunchBar or Alfred. The swipe down gesture brings this search anywhere. I have always used Spotlight a lot on my iOS devices, and it took a while to unlearn the swipe swipe swipe left gestures and replace them with swipe down, but it's great. I also find working menus much easier in iOS 7, as the clean lines make it fairly clear (for example, a contact's information is easier to scan for the right number or email address).
Disappointments for me are battery life and the zooming bits. I got used to the zoom effect within a few days, but I still find it excessive visual noise. I also had hoped a noticeable battery improvement, but maybe that will take app updates.
But my favorite part of iOS 7 is actually outside of iOS 7 proper: it's OmniFocus 2 for the iPhone. It rivals Day One for greatest app ever. It's perfect. The circles, colors, images, buttons: the delight and the efficiency, the fun and the geekery, are one.